OF VOLCANIC ROCKS. 75 



logically absurd to assume the non-existence of a foundation consisting of rocks differ- 

 ing in origin from sedimentary rocks ; and the conclusion is inevitable, that it must be 

 composed of rocks which were generated by the solidification of such masses as formed 

 part of the primordial substance of the globe, next to the surface. Probabilities 

 accumulate to point towards the assumption that very silicious granite composes that 

 foundation, together with such gneissoid rocks as must be supposed to have been formed 

 in an incipient sea of very high temperature, and under the pressure of the superin- 

 cumbent atmosphere of aqueous vapors, at a time when the cooling of the globe had 

 advanced far enough to allow of their first condensation, and when pressure, water, 

 and heat could cooperate to produce on its surface similar effects to those which the 

 same agencies arc supposed to have wrought in later periods at an ever-growing dis- 

 tance from it. The most probable mode of these ancient processes has been pointed 

 out by Daubree. The true nature of the foundation rock can, of course, not be 

 positively known, but must remain a matter of conjecture. No solution of this 

 problem will, however, be more satisfactory than that which is based on the hypoth- 

 esis of Sartorius, because it is in harmony with all the phenomena of vulcanism. 

 The probabilities in favor of a granitic foundation are, therefore, very great, while 

 no valid objection has yet been raised against its having the nature indicated. 



It is further argued that, the position of granite being always in the midst of 

 sedimentary, or of such foliated ciystalline rocks as are connected with the former by 

 gradual passage, and often conspicuously above such rocks, the only material from 

 which it could be generated are the sedimentary rocks themselves. The arguments 

 brought in evidence of these assertions are among the most potent which can be 

 adduced against them. Granite, in Norway, is superposed on Laurentian rocks turned 

 upon their edges, and, as no channel can be found by which it ascended, it is argued 

 that it must have been generated by metamorphism in situ. But as, on the other hand, 

 it has justly been remarked, that no position is more favorable for metamorphic action 

 than that of upturned strata, it is difficult to comprehend why the Laurentian rocks, 

 which were evidently nearer to the source of heat, should not have been metamor- 

 phosed in a much higher degree than the overlying granite. The same objection may 

 be made in other instances, as in the case of the Huronian and Laurentian rocks in 

 Canada, which were found to be overlain by granite thousands of feet in thickness. 

 The negative evidence, apparently afforded by the fact that no channels have been dis- 

 covered through which the granite could have protruded, is of very slight value, if it 

 is taken into consideration how rare are the instances where the conducting channels 

 can be seen, in the case of overflows of volcanic rocks, or even in that of currents of 

 lava. There are a few more instances known where granite can be distinctly observed 

 to overlie the upturned edges of stratified rocks, and must, therefore, have overflowed 

 the same in a liquid state. In some other cases, as in those of the extensive granitic 

 areas of Bessarabia and Western Australia,, the true geological position of granite 

 escapes observation ; while, in a number of others, it has the appearance of an intrusive 

 mass. 



The strongest objection which may be made to the eruptive nature of 

 granite relates to its lithological characters, which are different from those of the 



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